Field of the Invention
The invention relates to a substrate or a bulk crystal of semiconductor material used to produce semiconductor wafers for various devices including optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as transistors. More specifically, the invention provides crystals of group III nitride such as gallium nitride. The invention also provides various methods of making these crystals.
Description of the Existing Technology
This document refers to several publications and patents as indicated with numbers within brackets, e.g., [x]. Following is a list of these publications and patents:                [1] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615.        [2] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,132,730.        [3] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,160,388.        [4] K. Fujito, T. Hashimoto, S. Nakamura, International Patent Application No. PCT/US2005/024239, WO07008198.        [5] T. Hashimoto, M. Saito, S. Nakamura, International Patent Application No. PCT/US2007/008743, WO07117689. See also US20070234946, U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.        [6] D'Evelyn, U.S. Pat. No. 7,078,731.        
Each of the references listed in this document is incorporated by reference in its entirety as if put forth in full herein, and particularly with respect to their description of methods of making and using group III nitride substrates.
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives. However, the majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide because GaN substrates are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
To solve fundamental problems caused by heteroepitaxy, it is indispensable to utilize crystalline group III nitride wafers sliced from bulk group III nitride crystal ingots. For the majority of devices, crystalline GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow GaN crystal ingots. Currently, the majority of commercially available GaN substrates are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is one of vapor phase methods, which has difficulty in reducing dislocation density less than 105 cm−2.
To obtain high-quality GaN substrates for which dislocation density is less than 105 cm−2, various growth methods such as ammonothermal growth, flux growth, high-temperature solution growth have been developed. Ammonothermal method grows group III nitride crystals in supercritical ammonia [1-6]. The flux method and the high-temperature solution growth use a melt of group III metal.
Recently, high-quality GaN substrates having dislocation density less than 105 cm−2 can be obtained by ammonothermal growth. Since the ammonothermal method can produce a true bulk crystal, one can grow one or more thick crystals and slice them to produce GaN wafers. In the ammonothermal growth, bulk crystals of GaN are grown on seed crystals. However, since GaN or other group III nitride crystals do not exist in nature, one must fabricate GaN seed crystal with other method.
It is difficult to grow a seed crystal quickly that is suitable for use in ammonothermal bulk growth. Most methods today rely on seeds taken from a crystal formed by ammonothermal growth. Seeds thick enough for use in ammonothermal growth that are produced by e.g. vapor phase epitaxy typically crack, especially on a nitrogen-polar face of a crystal (such as the c-plane face). Consequently, while people may have tried obtaining seeds for ammonothermal crystal growth by forming the seeds in a faster-growth method, people have met with limited success in producing seeds via a method in which crystals grow faster than in an ammonothermal process.
This invention discloses group III nitride crystal which may be used for seed crystals in the ammonothermal bulk growth. In addition, this invention discloses methods of fabricating group III nitride crystals, which may be used for seed crystals in the ammonothermal bulk growth. Also this invention discloses a method of growing bulk crystals of group III nitride in supercritical ammonia using the group III nitride crystals as seeds.